Conventional Gilbert frequency mixers have balanced indexes and high port isolation, and operate reliably. However, as the requirement for single-chip of RF receivers increases and the RF technology advances, sometimes the performance of conventional Gilbert frequency mixers can not meet the present demand in many actual applications. For example, in the case that the frequency mixer stage employs an active frequency mixing structure, the flicker noise will have some adverse effect in zero IF receiver structure; if a passive frequency mixing structure can be employed, the flicker noise will be greatly decreased since a passive frequency mixer does not have quiescent current; in addition, the linearity of passive frequency mixer is usually higher than the linearity of active frequency mixer.
In the classic frequency mixing structure, as depicted in FIG. 1A, when RF voltage is converted into RF current in the transconductance stage, the transconductance value is limited and the conversion gain is low at low bias current, because the signal received by the receiver is generally low and the transconductance is only the transconductance of the input transistor in the conventional structure. It will be of great significance for performance improvement of the entire frequency mixer if higher transconductance value can be achieved at the same bias current in the transconductance stage by one approach. Based on that ideal, a novel transconductance circuit structure is designed successfully in the present invention; as a result, the transconductance value in the circuit of transconductance stage is greatly enhanced.
For the output stage of a passive frequency mixer, the input resistance of the transresistance amplifier in the output stage must be decreased as far as possible, owing to considerations of a series of problems, such as port isolation, linearity, and conversion gain, etc. In the present invention, the object is attained by means of a transresistance amplifier that employs a transconductance enhancement structure.